326 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



of ontogeny, although we can never point out where any stage 

 ends and another begins. To imagine that each single stage 

 of a part is present in the germ, as a distinct group of gem- 

 mules, seems to me to be a childish idea, comparable to the 

 belief that the skull of the young St. Laurence exists at Madrid, 

 while the adult skull is to be found in Rome. 



We are necessarily driven to such conceptions if we assume 

 that the transmission of acquired characters takes place. A 

 theory of preformation alone affords the possibility of an ex- 

 planation : an epigenetic theory is utterly unable to render any 

 assistance in reaching an interpretation. According to the latter 

 theory, the germ does not contain any preformed gemmules, 

 but it possesses, as a whole, such a chemical and molecular 

 constitution that, under certain circumstances, a second stage 

 is produced from it. For example, the two first segmentation 

 spheres may be regarded as such a second stage ; these again 

 possess such a constitution that a certain third stage, and no 

 other, can arise from them, forming the four first segmentation 

 spheres. At each of these stages the spheres produced are 

 peculiar to a distinct species and a distinct individual. From 

 the third stage a fourth arises, and so on, until the embryo is 

 developed, and still later the mature animal which can repro- 

 duce itself. No one of the parts of such an animal was originally 

 present as distinct parts in the egg from which it was developed, 

 however minute we may imagine these parts to be. If now an 

 inherited peculiarity shows itself in any organ of the mature 

 animal, this will be the consequence of the preceding develop- 

 mental stages, and if we were able to investigate the molecular 

 structure of all these stages as far back as the egg-cell, we 

 should trace back to the latter some minute difterence of mole- 

 cular constitution which would distinguish it from any other 

 egg-cell of the same species, and was destined to be the cause 

 of the subsequent appearance of the peculiarity in the mature 

 animal. It is only by the aid of some such h3^pothesis that we 

 can conceive the cause of hereditary individual differences and 

 the tendencies towards hereditary diseases. Hereditary epi- 

 lepsy would be intelligible in this way, that is, when the disease 

 is congenital and not due to the presence of microbes, as is 

 presumably the case with artificially induced epilepsy. 



The question now arises as to whether we can conceive the 



